NB: Not all colourblindness is genetic, and not all colourblindness is sex linked recessive. This post concerns the types which are (handy reference article).

WARNING! SCIENCE CONTENT!

The gene responsible for colourblindness is sex linked recessive, on the X chromosome. What that means, is that if it’s the only X present (as in XY males) the defect becomes active, but if you have two X chromosomes (as in XX females), the deficiency has to be on both to be active; if it’s only on one, the non-deficient genes take precedence.

So how do come about colourblind females? There are a few ways, but the most common is a female with both a colourblind father and a colourblind mate, which I’ll explain through thusly.

Take a female with a colourblind father. He has sex chromosomes XY; and the X being deficient renders him colourblind. This female, having two X chromosomes (“XX”), obtained one from each of her parents. Her father’s X being his sole deficient one, and either of her mother’s healthy Xs. As the defective gene is recessive, the mothers working genetic code takes dominance over the fathers defective genes, and this girl would exhibit normal colour vision.

Now lets say this girl was to mate with a colourblind male (XY, with colour deficient X). Her “XY” male children will receive their fathers “Y” chromosome, and an “X” from the mother. His “X” has a 50% chance of being defective (from the mother’s father: the boy’s colourblind grandfather), and a 50% chance of being healthy (from the mother’s mother: the boy’s normal visioned grandmother). Her male children, therefor, have a 50% chance of normal vision. Even with both parents carrying the defective gene he only has a 50% chance of being colourblind!

If, however, we were to suppost this girl had a female child. This XX female would receive the father’s deficient X chromosome, and (like the male XY child) receive one of her mother’s XXs at random. Resulting in a 50% chance of getting defective Xs from both her mother and her father. This means a female child has exactly the same chance - 1 in 2, 50:50 - of being colourblind as her brothers; as her XY chromosomal siblings.

So when this mother gives birth, there’s a 50% chance the child is colourblind, and a 50% chance that child is XX female. A 1 in 4 chance of making a girl; colourblind.

But what if we want to be certain the female children are colourblind (and all the male children too). To achieve this we require a colourblind female child from the previous scenario: with her two deficient X chromosomes. We then pair this colour deficient girl with yet another colourblind male. Their offspring, given all the possible donor “X” chromosomes are deficient, will all be colourblind; every single boy and girl.

So how many people are colourblind then? Given it’s seeming propensity to pass itself down generations. Well, I’ll leave that to Wikipedia. But needless to say, it runs in families; and when you find one, you can almost be certain there are more in the area!

So that’s it, how one gets colourblind. If you’re curious as to what colourblindness is, or what type of colourblindness you are; check out my earlier post on those very subjects.

Thanks for reading; helping the spread of the truth about colourblindness.

Extra reading for studious types
There is, as there is always, much more to the story. For those of you interested, is that it’s not actually 100% certain those final children are colourblind. Have you wondered why I’ve been saying “XX females” and “XY males”? It’s because not everyone’s born with only two sex chromosomes. Some are “XXY”, others “XXX”. Some, even, are “XYY”. But it doesn’t stop there, some people are “XXXY”, others “XXYY”, “XXXYY”, “XXXXY” and even “XXXXX”. Some have “XXY” in some of their cells, and “XY” in the rest. Each variation of X’s and Y’s has it’s a “syndrome” which is typical of those with those genes. There even exists those with a solitary “X”. Most of these variations have a reduced IQ, as compared with their XX and XY siblings, yet some are on par.

Colourblind genes resulting in better vision? The jury is out on this one, but there’s a lot of talk that some humans could be tetrachromats; that is, they would have four different colour receptor cells in the eye, whereas a typical human has only three. Curiouser and curiouser.